1. Field of the Invention
The present invention relates to a head position control method and disk storage device for controlling the position of a read head or read/write head on a rotating storage disk, and more particularly, to a head position control method and a disk storage device mounted with two or more heads.
2. Description of the Related Art
A disk storage device has a disk for storing data, a motor for rotating the disk, a head for recording and regenerating information onto the disk, and an actuator for moving the head to a target position on the disk. Typical disk storage devices of this kind are a magnetic disk device (hard disk drive, or HDD), or an optical disk device (DVD-ROM or MO).
FIG. 26 is a compositional diagram of a conventional head position control system, and FIG. 27 is an illustrative view describing the eccentricity of the disk. In a magnetic disk device, a positional signal for detecting the head position is recorded onto the disk. The positional signal is constituted by a servo mark and track number, and offset information. Using the track number and offset information, the present position of the head can be ascertained.
As shown in FIG. 26, the control system for controlling the head position according to the present position and the target position calculates the positional error (r−y) between the target position r and the present position y, in a calculator 106, and inputs same to a controller C. The controller C is constituted by a commonly known PID controller or observer, which calculates a current for eliminating the position error and outputs the same to a plant actuator P. The actuator P is driven and the present position y is output from the head provided on the actuator P.
Moreover, in order to follow disk eccentricity, an eccentricity correction current corresponding to the selected head (disk face) is output from a table 108 storing eccentricity correction currents for each head, and is added to the command current of the controller C and supplied to the plant P.
In other words, the controller C determines the differential between the positional information and the target position, performs a calculation according to the amount of positional deviation thereof, and supplies a drive amount for driving the actuator P, for example, a current in the case of a VCM (voice coil motor), or a voltage in the case of a piezo-electric actuator, or the like.
As illustrated in FIG. 27, in this disk device, eccentricity, in other words, misalignment between the disk center and the motor center causing the disk to rotate, causes a problem. More precisely, a problem is caused by misalignment between the rotational center when recording a servo signal onto the disk 110, 112, and the rotational center of the motor of the disk device. A servo signal is recorded respectively onto the disks 110 and 112. When this signal is recorded, the eccentricity between the two disks is “0”.
However, misalignment of the two disks 110, 112 may cause these two disks 110, 112 to deviate from the rotational center of the motor, as illustrated in FIG. 27. A problem then arises in that the amount of such deviation will be different in the respective disks 110, 112. The cause of eccentricity of this kind may be from external shocks, thermal deformation, or the like. Moreover, in a system where the recording of the servo signal is performed on individual disks before assembling the device, whereupon the device is then assembled, eccentricity will invariably occur and the differential in eccentricity between disks will be large.
The differential in the eccentricity between disks means that when switching heads, the eccentricity following of the switched head is necessary. Conventionally, various types of techniques has been proposed in order to facilitate the operation of following eccentricity when switching heads.
For example, Japanese Laid-open Patent No. 2000-21104 proposes a method where, in the case of a device having eccentric misalignment between heads, an internal variable for generating an eccentricity correction current within a controller is changed when the head is switched. If an eccentricity correction control system such as that described in this example is applied, then it can be possible to suppress eccentricity, even in the case of a device having large eccentricity, and even if there is a differential in the amplitude and phase of the eccentricity between the inner and outer perimeters of the disk.
Moreover, Japanese Laid-open Patent No. 2001-283543, and others, propose a method for dealing with changes in the eccentricity and phase of the eccentricity correction current at the inner and outer perimeters occurring when the disk eccentricity is large.
Nevertheless, conventional technology, including the proposals described above, has been devised with respect to the current after head switching, or the positional error between heads. However, this technology does not take into account initial velocity or initial current immediately after head switching.
Initial velocity and initial current immediately after head switching are now described with reference to FIG. 28 and FIG. 29.
FIG. 28 shows an example where there is a differential in the eccentricity trajectory between head 0 and head 2, and depicts the movement of the servo signal on the face of a head 2 over a separate disk, while controlling the position of a head 0 over a certain track. In order to simplify the explanation, in FIG. 28, the face of head 0 will be assumed to have “0” eccentricity, and only the face of head 2 will be regarded as having eccentricity. The current, velocity, and position of the face of head 2 are thus depicted.
As shown in FIG. 28, the servo signal on the face of head 2 follows a sinusoidal trajectory having the same frequency as the rotational frequency, with respect to the servo signal on the face of head 0. A similar explanation can be used in cases where both sides of the disk have eccentricity. In this state, head switching is performed. As shown in FIG. 29, after switching heads, the eccentricity correction current is switched from eccentricity correction for the face of head 0 to eccentricity correction for the face of head 2. Thereby, a step of magnitude u0 is generated in the relative current U. Moreover, there will be an initial velocity of relative velocity V0 corresponding to the differential between the sinusoidal trajectories of head 0 and head 2. The relative position will also change by X0.
In the prior art, when switching heads, control of switching is performed by addressing only the relative position X0 or the eccentricity correction current themselves, as illustrated in FIG. 29. However, the current u0 and initial velocity V0 are not taken into account.
In the present invention, on the other hand, initially the existence of the initial velocity V0 is investigated. When switching heads, seek control is performed. However, in a conventional control system, it is assumed that the initial velocity at head switching is 0, or that it is the same as the velocity of the previous head. “Seek control” involves performing control in such a manner that the distance to the target position becomes “0” and the speed upon reaching the target position is also “0”. However, since the initial speed is not “0”, then a corresponding disparity occurs. This disparity creates a fluctuation upon arriving at the target position, and since this fluctuation takes time to be absorbed, it causes a lengthening of the seek time.
Next, there is the problem of the step u0 in the eccentricity correction current. If the differential in the eccentricity trajectory is large, then this step u0 will also be large. When switching heads, this sudden change in the current stimulates the resonance of the actuator, and is a cause of fluctuation. As a means of suppressing this resonance, a current is output to the actuator via a filter, such as a notch filter, or the like. However, even if several notch filters are used, the resonance frequency component cannot be reduced fully to zero, and the notch filter waveform cannot provide 100% correction of the actuator resonance characteristics. Consequently, if there is a sudden step in the initial current u0 as illustrated in FIG. 29, fluctuation may occur, which in turn becomes a cause of delay in the seek time.
Therefore, it is an object of the present invention to provide a head position control method and disk device for reducing lengthening of the seek time due to the initial velocity V0 upon switching heads.
It is a further object of the present invention to provide a head position control method and disk device for reducing lengthening of the seek time due to the initial current u0 upon switching heads.
Moreover, it is yet a further object of the present invention to provide a head position control method and disk device for recording or reproducing data by moving an actuator to a target position at higher speed, after switching heads.